1. Technical Field
This invention relates generally to encoding pictorial imagery for reproduction on binary display and/or printing systems, and more particularly to suppressing false density contours caused by an insufficient number of output gray levels in the reproduction system.
2. Background Art
Binary displays and printers are capable of making a mark, usually in the form of a dot, of a given, uniform size and at a specified resolution in marks per unit length, typically dots per inch. It has been common to place the marks according to a variety of geometrical patterns such that a group of marks when seen by the eye gives a rendition of an intermediate color tone between the color of the background (usually white paper stock) and total coverage, or solid density.
Continuous tone images are simulated by organizing groups of sub-elements into j.times.k matrix halftone cells, where j and k are positive integers. The halftone cells have gray level capabilities equal to the number of sub-elements in the cell plus one.
Halftone image processing algorithms are evaluated in part by their capability to deliver a complete gray scale at normal viewing distances. The capability of a particular process to reproduce high frequency renditions (fine detail) with high contrast modulation makes that procedure superior to one which reproduces such fine detail with lesser or no output contrast.
Another measure of image processing algorithm merit is the tendency to produce visual details in the output image that are not part of the original image, but are the result of the image processing algorithm. Such details are called artifacts, and include moire patterns, false contours, and false textures. Moire patterns are false details created most often by the beating between two relatively high frequency processes resulting in a signal whose spacial frequency is low enough to be seen by the viewer. False textures are artificial changes in the image texture which occur when input gray levels vary slowly and smoothly and the output generates an artificial boundary between the textural patterns for one gray level and the textural patterns for the next gray level. False contours are the result of gray scale quantization steps which are sufficiently large to create a visible density step when the input image is truly a smooth, gradual variation from one to the other.
Briefly, several of the commonly used processing algorithms include fixed level thresholding, adaptive thresholding, orthographic tone scale fonts, and electronic screening. The present invention is concerned with the latter, electronic screening, and with suppressing false contours in screened images.
There are many formats for electronic halftone cells at various screen angles and screen frequencies. FIG. 1 shows a sequence number matrix for a halftone screen design at 97 lines per inch ruling with a 400 dpi at 400.times.400 addressable points per square inch. There are eighteen levels of gray (17 sub-elements plus white) associated with each halftone cell which is enclosed within the thick lines in the figure. The numeral that is associated with each sub-element within a halftone cell is the sequence number that the sub-elements within the cell is filled sequentially as the density in the cell increases. Each halftone cell is stacked to form a halftone screen with screen angle of 104.degree..
A problem exists with the number of density levels attainable with a limited resolution and acceptable screen frequency. Eighteen levels is not generally sufficient; more gradations being preferred to suppress false contouring. One way to get more gray levels is to include more sub-elements in a cell by increasing the size of the cell, but this reduces the number of lines per inch and decreases the screen frequency to a visible level. That is, the more sub-elements in a cell, the more gray levels can be reproduced, but larger cells become more observable and objectionable.